Radioactive heat production in the Idaho Batholith and in the Alamosa River Stock in the San Juan Mountains correlates with the degree of hydrothermal alteration of plutons by meteoric groundwater. Fission-track mapping shows that as much as half of the U in whole-rock samples of the most-intensively-altered zone of the Alamosa River Stock is associated with secondary minerals. The hydrothermally-altered, shallow plutons in the Idaho Batholith contain significant amounts of U in secondary minerals whereas unaltered, deeply-emplaced plutons contain virtually all of their U in primary accesory minerals. The relative distributions of U and Th measured by gamma-ray spectrometry, the associations of U observed by fission-track mapping and a depth-dependence in thermally-induced hydraulic fracturing suggest a depth-dependent geochemical process that could produce an upward increase in the abundance of U and Th in batholiths. The process adds U and Th to chemically-receptive secondary minerals during alteration of plutons by convecting groundwater. ¿ American Geophysical Union 1987